1. Field of the Invention
The invention pertains generally to computers. In particular, it pertains to managing device power states in computers.
2. Description of the Related Art
In an effort to conserve energy, computer systems have increasingly begun to implement low-power states, in which various techniques are used to reduce power consumption. This is especially true in battery-powered systems, in which reduced power consumption produces increased operational time between battery charges. These power reduction efforts have been targeted at various levels of the computer systems, ranging from the entire system to individual devices. Various power states have been defined to provide common terminology in power reduction efforts. States S0 through S5 define system level power states, with S0 being fully operational. S1-S5 define non-operational levels of progressively decreasing power consumption and progressively longer times to recover full operation when returning to the S0 state. States D0 through D3 define similar states for devices, such as disk drives, displays, etc., with D0 indicating fully operational and D1-D3 indicating progressively d e creasing power consumption and progressively longer times to recover full operation when returning to the D0 state. These terms define just one operational state for a device, the DO state, with the remaining states defining various levels of low-power consumption in a non-operational mode. Thus the power management system, whatever form it might take, primarily needs to choose between operation and non-operation for each power-managed device at any given time. Secondary considerations may then be evaluated to determine how quickly the device must respond when it is returned to an operational status, and that evaluation determines which low-power state to enter.
However, some devices are suitable for various levels of reduced-power performance in the operational state. For example, the backlight of a liquid crystal display (LCD) device can be turned off without disabling the pixel control of the display. Similarly, the transmission circuits of a modem could be placed in a low power state until a transmission is to be made, while leaving the receive circuits fully operational to receive any incoming data. These power controls are generally placed in the hardware, and are based on hardware timers and hardware signals that signify an event that must be responded to. The software generally has no insight into these partial-power states, and no control over them. Since these are independent states with independent logic, there is no overall control of the device power management system. Any device interdependencies (i.e., one device won""t operate correctly while another is in a given low power state) are not comprehended, and the operating system cannot provide comprehensive power management control because it is unaware of the current operating levels of all the various devices.
Unfortunately, the power management systems of most computers are not set up to provide such comprehensive power control over device operational states. Both the hardware and software to provide this comprehensive control are missing in a conventional system.